Ultrasonic scanning systems acquire data and provide medical images of the interior of patients using the acquired data. In general the systems use transducers to transmit ultrasonic waves in the order of several megahertz in frequency into a subject or a patient. "Echo" signals are received and used as the data for the images. In general the transducer is positioned juxtaposed to the patient's body. The intensities of the received echo signals are measured. The location of the echo generating material (i.e. boundaries of organs and the like) is determined by the time required for the echo to return after the original signal is transmitted. The acquisition of the intensity data correlated to location in the body enables obtaining intensity values for image pixels corresponding to body locations as is required for providing images.
A long standing problem encountered in the acquisition of ultrasonic imaging data is the attenuation of the signals due to the distance traversed by the signals. Therefore, echoes received from further within the body have much less intensity than echoes received from the same type of tissue boundaries which are close to the surface of the body. For imaging purposes the echoes coming from the same type of tissue boundaries should have the same intensity without regard to the distance. The attenuation thus amounts to a distortion. Normally to overcome this distortion, ultrasonic systems are provided with TGC equipment to correct the intensity of the echoes so that the intensities are the same for echoes received from tissues deep within the body as they are for similiar tissues close to the surface of the body.
The echo signals are relatively weak and require amplification. In practice the gain of the amplifier is varied by the TGC equipment to overcome the effect of the attenuation of the echo signal due to depth. Care has to be taken in the use of the TGC equipment to prevent or at least minimize the adverse effect of the varying gain on actual data.
The prior art TGC equipment requires the doctor to make the adjustments of the amplifier gain by eye based on his view of the image. Thus, the doctor attempts to vary the amplifier gain while looking at the image to overcome the depth caused attenuation. Originally the attempt at compensating the image intensity for depth was accomplished with analog signals used to control the amplifier gain. More recently the amplifier gain has been controlled using digital signals stored as preprogrammed charts. See for example, U.S. Pat. No. 4,356,731 which issued on Nov. 2, 1982 and teaches a method for generating TGC signals for use in ultrasonic scanners and the like. More particularly the patent teaches utilizing a preprogrammed chart stored in a RAM to define the amplifier gain function. Thus, the doctor operates the control panel to generate address signals causing the preprogrammed chart of the RAM to provide the amplification factor on a digital basis in order to compensate for the depth caused attenuation.
A problem with the prior art systems is that the operator of the system (i.e. the doctor or clinician making the test) has to manipulate the ultrasound scanner while operating manual controls to vary the amplification gain to obtain intensity signals independent of distance. Thus, the doctor has to operate the scanner, operate control buttons and observe the image all at the same time. Accordingly with the prior art systems it is difficult to make gain corrections speedily and in a reliable manner.
Therefore there is a need for ultrasound imaging systems which automatically provide TGC for ultrasound imaging without the necessity of operating controls to correct for the changes in the intensity of the echo signal due to the depth travelled by the transmitted ultrasound signals.
A related problem faced by all TGC circuits and methods used with ultrasound systems is to assure that the time gain compensation does not destroy or adversely effect the data which after all comprises signals of varying intensity.
Accordingly improved TGC circuits and methods are required which can vary the ultrasound system amplification on a real time basis to compensate for attenuation caused by the distance travelled through the subject's body while preserving the actual data.